1. Field of the Invention
This invention relates to structure for the entry of data into a computer or a graphics display device and in particular to a tablet which is capable of translating the particular position touched on the tablet to digital representation of the X and Y coordinates of that position.
2. Prior Art
A number of different devices have been proposed for controlling the movement of a cursor upon a visual display. Among the various cursor controls suggested is a structure comprising four sets of switches, each controlling cursor movement in one of four orthogonal directions as disclosed in U.S. Pat. No. 4,313,113 on an invention of David Thornburg.
Additional structure for the entry of two dimensional graphic data into a digital system such as a communication system is disclosed in U.S. Pat. No. 4,079,194 issued Mar. 14, 1978 on an application of Kley entitled "Graphical Data Entry Pad". Kley discloses a resistive grid area to which a conductive layer may be selectively contacted so that graphical data can be generated by external electronic circuitry which measures resistance. The resistive grid area is formed by a pattern of resistive inking on a base and is separated from the conductive layer by a foamed thermal plastic insulating separator. High resistance bands surround the grid area so as to linearize its output value. The structure of the '194 patent comprises a plane rectangular resistive grid area with four edges and of moderate resistance material and a conductive layer spaced from but parallel to the resistive grid area such that the resistive grid area or the conductive layer can be displaced in the direction of the other so as to make electrical contact at a selected point. The four edges of the resistive grid area are contacted by resistive structures having an effective resistance parallel to the edges of the resistive grid area substantially higher than that of the resistive grid area. Four conductors are provided to connect on a one-to-one basis, the resistive structure connecting the edges of the resistive grid area to external circuitry for determining the X and Y coordinates of the point of contact between the resistive grid and the conductive layer. In implementing the structure, the resistive grid is separated from the conductive layer by "a large number of depending projections 24 . . . formed closely packed all along the lower surface of the top cover 12, the projections 24 extending so as to penetrate into the insulating separator 20" between the resistive grid area and the conductive material. (U.S. Pat. No. 4,079,194, Column 2, lines 23 to 26). The insulating separator 20 between the conductive layer and the resistive material is formed of an insulating therma plastic or other resilient and flexible material of a relatively low density. Upon pressing the top cover 12, the projections 24 pass through the insulating material into contact with the resistive material on the base pad 22. In an alternative embodiment, the top cover has no depending projections 24 but rather the insulating separator 20 "is of an open-work type, being provided throughout its area with a multipicity of openings 58 which extend therethrough." (U.S. Pat. No. 4,079,194, Column 5, lines 5 to 8). Depressing the top cover 12 causes the insulating separator 24 to compress so that the top cover 12 touches the resistive material on the top of the base pad 22 through one or more of the openings 58.
The structure disclosed by Kley has the undesirable characteristic that the X, Y coordinates of a position contacted on the cover 12 can only be determined within the accuracy of the dimensions of the projections 24 or the openings 58 in the separator 20 between the conductive material on the bottom surface of the top cover 12 and the resistive material on the top surface of the base pad 22.
Kley in U.S. Pat. No. 4,214,122 issued July 22, 1980 discloses a similar rectangular structure wherein the arrangement of conductive strips relative to the resistive grid area is selected so as to improve the linearity of the voltage gradients in the rectangular graphic area.
Additional prior art patents disclosing graphical entry devices employing rectangular resistive sheets or planes include U.S. Pat. Nos. 2,900,446, 3,304,612, 3,522,664, 3,591,718, 3,699,439, 3,798,370 and 3,806,912.
U.S. Pat. No. 3,522,663 discloses a flexible member coated with a conductive layer which is positioned above a base member which is coated with a resistive material so that the location of a point at which the two members are pressed together can be determined by measuring the resistances from the edges of the layer to the resistive material at that point. Again the output signal is less linear than desired.
Another technique for deriving X, Y position information for input into a dispay system is disclosed in U.S. Pat. No. 3,541,541 disclosing an indicator control mechanism for movement by hand over any surface. The mechanism generates position signals which cause a cursor displayed on a cathode ray tube to move to a corresponding position. The mechanism contains X and Y position wheels mounted perpendicular to each other which rotate according to the X and Y movements of the mechanism and which operate rheostats to send signals along a wire to the computer which controls the CRT display.
U.S. Pat. No. 3,705,956 discloses an electromagnetic sensor held in a spaced relation to a grid network having a plurality of conductors defining a Gray Code pattern. Each of the conductors corresponds to a grid in the Gray Code and the grid network is characterized by differences which correspond to the differences in the bits of the Gray Code. As current is passed sequentially through each of the conductors, a voltage is induced in the sensor. The location of the sensor with the respect to the grid network is determined by detecting the phase of the voltage induced in the sensor.
An additional structure for this purpose is disclosed in U.S. Pat. No. 3,911,215 issued Oct. 7, 1975 on an application of Hurst, et al., entitled "Discriminating Contact Sensor". The sensor comprises two juxtaposed electrical potential carrying sheets, at least one being flexible, separated from each other but permitting contact therebetween when an object of specified radius of curvature is pressed against the flexible sheet. The separation of the sheets is accomplished by discrete small buttons of insulation preferably mounted on the flexible sheet, with the spacing and the height of the buttons determining the largest radius of curvature to which the sensor will respond. Contact is made only by depression of the flexible sheet with a writing instrument and not by any portion of a writer's hand.
U.S. Pat. No. 4,319,078 issued Mar. 9, 1982 on an application of Yokoo, et al., discloses apparatus for detecting X and Y coordinates of the input points consisting of a first parallel electrode group and a second parallel electrode group opposing and perpendicular to the first parallel electrode group. A pressure conductive rubber sheet is interposed between the first and second parallel electrode groups. This sheet is rendered conductive upon application of pressure to the input surface. An electrical detector is provided to detect currents flowing through first and second resistors contacting terminal ends of the first and second parallel electrode groups, respectively.
Additional entry structures are provided from Sierracin using a membrane switch denoted as Transflex (a trademark of Sierracin). This structure comprises a film coated with a transparent electrically-conductive layer of either gold or indium tin oxide. The material is mounted on the face of a cathode ray tube and the alphanumeric displays on the tube are read through the material. The entire switch pad area is conductive and can be touched anywhere within the designated area of the graphic display to obtain a desired response.
In one product produced by Sierracin, two pieces of an insulating film are each coated on one surface with conductive layers. Each layer then is etched to form parallel isolation lines between parallel conductive lines. The two films are then mounted such that the conductive lines on the two films are orthogonal and the conductive lines on one film are separated from the conductive lines on the other film by a small air gap. Separation is accomplished through the use of edge gaskets and/or interlayers. By applying a slight pressure, the films are brought into electrical contact with each other allowing current to flow in a specific circuit. In one embodiment, the usual method of retrieving information is to alternately drive one of the sheets of the panel with either a constant voltage or current source while using the other sheet as a sensor. The resulting X and Y signals are then scaled, filtered and digitized to meet system requirements. Whether a particular output signal is from the X direction or the Y direction depends upon which sheet is used as a sensor and which is used as the input signal source. The output signal must be taken through a pull down resistor which is large in value compared to the resistance of the sheet used as the sensor.
FIGS. 5a, 5b, 5c and 5d illustrate the circuitry associated with this prior art sensing sheet.